Field
Embodiments described herein generally relate to a method and apparatus for processing semiconductor substrates, and more particularly, to modifying a work function of a metal film processing.
Description of the Related Art
In a field-effect transistor (FET), threshold voltage is the minimum gate-to-source voltage differential required to create a conducting path between the source and drain terminals and thereby turn the FET on. That is, when the gate voltage, i.e., the voltage applied to the gate of the FET, is above the threshold voltage, there are sufficient electrons in the channel of the FET at the oxide-silicon interface to create a low-resistance channel in which charge can flow between the source and the drain. Conversely, when the gate voltage is below the threshold voltage, the transistor is off, in which case there is no current between the source and the drain of the transistor, except for leakage current.
Since the threshold voltage determines the requirements for turning a transistor on or off, precise control of the threshold voltage is important in designing a properly operating transistor. As is well-known, threshold voltage of a transistor is a function of the thickness and dielectric constant of the transistor gate dielectric. Consequently, one technique for designing a transistor to operate with a particular threshold voltage involves scaling the gate dielectric thickness downward, i.e. reducing the dielectric thickness proportional to reductions in other important transistor dimensions, such as transistor length and width.
However, as transistors are reduced in size with each technology node, precise control of threshold voltage by scaling gate dielectric thickness can be impracticable. Specifically, with a linear reduction of the thickness of the conventional oxide/oxynitride dielectric layer in some FETs, there is an exponential increase in gate leakage, resulting in increased power consumption. Furthermore, the thickness of the dielectric layer is now close to a few atomic layers, raising reliability concerns. Thus, adjusting threshold voltage in a transistor by continued downward scaling of gate dielectric thickness is problematic.
Threshold voltage is also a function of the thickness and work function of the gate conductor material. Thus, another technique for controlling the threshold voltage of a particular transistor involves using a gate conductor material that has a work function close to a target value, and, in some cases, selecting the deposited thickness of the gate conductor material to fine-tune the effective work function of the gate conductor material to the target value. For example, titanium nitride (TiN), having a work function value of about 4.6 eV, is commonly employed as a gate conductor material in some metal gate structures. Since the work function of a deposited TiN layer varies with the thickness of the deposited layer, the work function of TiN can be tuned from about 4.5 eV to about 4.7 eV.
However, as transistors are reduced in size with each technology node, reducing the thickness of such a TiN layer to decrease the effective work function of the TiN layer is not be a viable option. For instance, with TiN layers in metal gate structures currently in the 10-20 Å range, a further decrease in thickness may result in a TiN layer of only a few atomic layers. Thus, adjusting threshold voltage in a transistor by reducing metal gate material thickness is also problematic.
Accordingly, there is a need in the art for improved techniques to modify threshold voltage of a transistor.